Control of thermoelectric properties in Mn-substituted ${\mathrm{Fe}}_2\mathrm{TiSi}$ epilayers

We experimentally study the Mn substitution effect on thermoelectric properties in a full-Heusler alloy, ${\mathrm{Fe}}_2\mathrm{TiSi}$. By employing molecular beam epitaxy, homogeneous and $L{2}_1$-ordered ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$ epilayers are achieved. The lattice constant, the saturation magnetic moment, and electrical resistivity are intentionally controlled with increasing Mn substitution. Notably, we find that the sign of the Seebeck coefficient is varied from positive to negative at around $x=0.2$. On the basis of the first-principles calculations, we qualitatively understand that the observed variation in the electrical and thermoelectric properties arises from the change in the electronic band structure near the Fermi level, dominated by Fe and Mn atoms in ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$.

Figure 1

(a) $\theta -2\theta$ XRD patterns for the ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$ epilayers. (b) $\varphi$-scan measurements of the (111) plane for the ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$ epilayers. (c) The lattice constant of the ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$ epilayers estimated from (a). The inset shows the RHEED pattern for the ${\mathrm{Fe}}_2{\mathrm{Ti}}_{0.42}{\mathrm{Mn}}_{0.58}\mathrm{Si}$ layer grown at $350{}^{\circ }\mathrm{C}$.

Figure 2

(a) HAADF-STEM image with EDX line profiles of ${\mathrm{Fe}}_2{\mathrm{Ti}}_{0.42}{\mathrm{Mn}}_{0.58}\mathrm{Si}/{\mathrm{MgAl}}_2{\mathrm{O}}_4$(001) and an enlarged image near the interface. (b) EDX elemental maps for ${\mathrm{Fe}}_2{\mathrm{Ti}}_{0.42}{\mathrm{Mn}}_{0.58}\mathrm{Si}/{\mathrm{MgAl}}_2{\mathrm{O}}_4$(001).

Figure 3

(a) Calculated total spin moment of ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$ with the presence of 6% Fe $\iff$ Ti disordering (red circles) and the experimental data of ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$ epilayers at 10 K (blue triangles). The inset shows $M-H$ curves for ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$ epilayers at 10 K. (b) Total and local spin moments of ${\mathrm{Fe}}_2{\mathrm{Ti}}_{0.13}{\mathrm{Mn}}_{0.87}\mathrm{Si}$ as a function of $y$ (Fe $\iff$ Mn disordering concentration) with the presence of 6% Fe $\iff$ Ti disordering. The gray dashed line indicates the experimental data of ${\mathrm{Fe}}_2{\mathrm{Ti}}_{0.13}{\mathrm{Mn}}_{0.87}\mathrm{Si}$ epilayers at 10 K.

Figure 4

(a) Temperature dependence of $\rho$ for the ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$ epilayers. The inset shows a micrograph of the fabricated Hall-bar device. (b) The values of $\rho$ at 300 K as a function of $x$ in the ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$ epilayers.

Figure 5

Room-temperature (a) Seebeck coefficient $S$ and (b) power factor (PF) of the grown ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$ epilayers.

Figure 6

Spin-resolved partial density of states (DOS) at the (A,C) sites (left column) and at the B site (middle column) of ${\mathrm{Fe}}_2{\mathrm{Ti}}_{1-x}{\mathrm{Mn}}_x\mathrm{Si}$ with the presence of 6% Fe $\iff$ Ti disordering. The right column shows the total DOS near the Fermi level. We assumed that Mn atoms occupy only the B site.